When an audio system is used in an environment with a varying ambient noise level, it is often necessary to adjust the output volume of the audio system to overcome the ambient noise level. For example, when listening to music in a vehicle, it is necessary to turn up the volume when the vehicle is driving on a rough road, and turn down the volume when the vehicle comes to a stop. For a message announcement system in a transportation station, such as airport or train station, it is desirable to turn down the volume of loudspeakers during early morning hours, and turn up the volume during busy hours. Likewise, when listening to music or message broadcast in a factory environment, it is desirable to increase the loudspeaker volume when machinery is running, and decrease the volume when machinery is idle.
In a typical automatic volume control system, a music signal from a music source is broadcast via a loudspeaker, and a microphone is used to detect an ambient room signal which includes the music portion and the ambient noise portion. When the original music signal is subtracted from the ambient room signal detected by the microphone, the resulting error signal approximates the ambient noise. However, such a method of measuring and compensating for the ambient noise level is inaccurate because the music signal is modified by the room conditions. In a house or apartment, the room conditions may include room size, furniture arrangements, number of people in the room, etc. In a vehicle, the “room” conditions may include the size of vehicle, seat configurations, and number of people present in the vehicle, etc. In addition, other factors such as phase delay from the speaker to the microphone, frequency response of various components of the audio system, etc., also affect the actual sound signal received by the microphone. The effect of sound modification due to the various conditions and factors is called the room transfer function. To obtain better results, an adaptive filter is often used in typical audio systems to simulate the room transfer function. When the music signal is passed through the adaptive filter, the filtered signal approximates the modified music portion of the ambient room signal received by the microphone. By subtracting the filtered signal from the ambient room signal detected by the microphone, a better approximation of the ambient noise is obtained, and volume of the music signal is adjusted to compensate the ambient noise. However, known adaptive filters still fail to adequately compensate for ambient noise automatically under varying conditions and environments.